// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// A "smart" pointer type with reference tracking.  Every pointer to a
// particular object is kept on a circular linked list.  When the last pointer
// to an object is destroyed or reassigned, the object is deleted.
//
// Used properly, this deletes the object when the last reference goes away.
// There are several caveats:
// - Like all reference counting schemes, cycles lead to leaks.
// - Each smart pointer is actually two pointers (8 bytes instead of 4).
// - Every time a pointer is released, the entire list of pointers to that
//   object is traversed.  This class is therefore NOT SUITABLE when there
//   will often be more than two or three pointers to a particular object.
// - References are only tracked as long as linked_ptr<> objects are copied.
//   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
//   will happen (double deletion).
//
// Note: If you use an incomplete type with linked_ptr<>, the class
// *containing* linked_ptr<> must have a constructor and destructor (even
// if they do nothing!).
//
// Thread Safety:
//   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
//   effectively a read-write operation.
//
// Alternative: to linked_ptr is shared_ptr, which
//  - is also two pointers in size (8 bytes for 32 bit addresses)
//  - is thread safe for copying and deletion
//  - supports weak_ptrs

#ifndef BASE_MEMORY_LINKED_PTR_H_
#define BASE_MEMORY_LINKED_PTR_H_

#include "base/logging.h" // for CHECK macros

// This is used internally by all instances of linked_ptr<>.  It needs to be
// a non-template class because different types of linked_ptr<> can refer to
// the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
// So, it needs to be possible for different types of linked_ptr to participate
// in the same circular linked list, so we need a single class type here.
//
// DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
class linked_ptr_internal {
public:
    // Create a new circle that includes only this instance.
    void join_new()
    {
        next_ = this;
    }

    // Join an existing circle.
    void join(linked_ptr_internal const* ptr)
    {
        next_ = ptr->next_;
        ptr->next_ = this;
    }

    // Leave whatever circle we're part of.  Returns true iff we were the
    // last member of the circle.  Once this is done, you can join() another.
    bool depart()
    {
        if (next_ == this)
            return true;
        linked_ptr_internal const* p = next_;
        while (p->next_ != this)
            p = p->next_;
        p->next_ = next_;
        return false;
    }

private:
    mutable linked_ptr_internal const* next_;
};

// TODO(http://crbug.com/556939): DEPRECATED: Use scoped_ptr instead (now that
// we have support for moveable types inside STL containers).
template <typename T>
class linked_ptr {
public:
    typedef T element_type;

    // Take over ownership of a raw pointer.  This should happen as soon as
    // possible after the object is created.
    explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
    ~linked_ptr() { depart(); }

    // Copy an existing linked_ptr<>, adding ourselves to the list of references.
    template <typename U>
    linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }

    linked_ptr(linked_ptr const& ptr)
    {
        DCHECK_NE(&ptr, this);
        copy(&ptr);
    }

    // Assignment releases the old value and acquires the new.
    template <typename U>
    linked_ptr& operator=(linked_ptr<U> const& ptr)
    {
        depart();
        copy(&ptr);
        return *this;
    }

    linked_ptr& operator=(linked_ptr const& ptr)
    {
        if (&ptr != this) {
            depart();
            copy(&ptr);
        }
        return *this;
    }

    // Smart pointer members.
    void reset(T* ptr = NULL)
    {
        depart();
        capture(ptr);
    }
    T* get() const { return value_; }
    T* operator->() const { return value_; }
    T& operator*() const { return *value_; }
    // Release ownership of the pointed object and returns it.
    // Sole ownership by this linked_ptr object is required.
    T* release()
    {
        bool last = link_.depart();
        CHECK(last);
        T* v = value_;
        value_ = NULL;
        return v;
    }

    bool operator==(const T* p) const { return value_ == p; }
    bool operator!=(const T* p) const { return value_ != p; }
    template <typename U>
    bool operator==(linked_ptr<U> const& ptr) const
    {
        return value_ == ptr.get();
    }
    template <typename U>
    bool operator!=(linked_ptr<U> const& ptr) const
    {
        return value_ != ptr.get();
    }

private:
    template <typename U>
    friend class linked_ptr;

    T* value_;
    linked_ptr_internal link_;

    void depart()
    {
        if (link_.depart())
            delete value_;
    }

    void capture(T* ptr)
    {
        value_ = ptr;
        link_.join_new();
    }

    template <typename U>
    void copy(linked_ptr<U> const* ptr)
    {
        value_ = ptr->get();
        if (value_)
            link_.join(&ptr->link_);
        else
            link_.join_new();
    }
};

template <typename T>
inline bool operator==(T* ptr, const linked_ptr<T>& x)
{
    return ptr == x.get();
}

template <typename T>
inline bool operator!=(T* ptr, const linked_ptr<T>& x)
{
    return ptr != x.get();
}

// A function to convert T* into linked_ptr<T>
// Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
// for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
template <typename T>
linked_ptr<T> make_linked_ptr(T* ptr)
{
    return linked_ptr<T>(ptr);
}

#endif // BASE_MEMORY_LINKED_PTR_H_
